More particularly when developing and operating internal combustion engines operated by lean fuels, it becomes more and more important to continuously determine acid, oxygen-containing nitrogen components such as nitrogen dioxide NO2. In addition to a more stringent exhaust gas legislation which leads to a maximum permissible emission limit for NO2, an accurate determination of nitrogen oxide fractions in the exhaust gas plays an important part for the subsequent treatment of exhaust gases.
It is generally known to provide a removal point in the exhaust gas pipe and a removal line for simultaneously measuring all exhaust gases, more particularly nitrogen oxide NOx and hydrocarbons HC.
From DE 33 39 073 A1 there are known a method and a device for analyzing gas mixtures, wherein individual partial gas flows are guided to individual analyzing devices, for example for nitrogen oxides NOx. The main gas flow is heated in front of a removal point for an exhaust gas sample for measuring hydrocarbons, thereafter guided to a cooler and filter, and of said main gas flow conditioned in said way, there are branched off the further partial gas flows for exhaust gas samples for measuring other constituents such as nitrogen oxides NOx.
At temperatures of approximately 190° C. which are necessary in order to avoid condensation of hydrocarbons, it is possible for there to occur a partial reduction of the nitrogen dioxide to be measured. Furthermore, there may occur a reaction at surfaces coming into contact with nitrogen dioxide, such as the removal line, bolted connections and the like. Above all, a surface reaction has been observed on steel surfaces, more particularly high-grade steel surfaces.